There have been stricter regulations each year on amounts of emissions of NOx (nitrogen oxides) and particulate matter (hereinafter abbreviated as PM) emitted from a diesel engine, along with CO (carbon monoxide), HC (hydrocarbon), and the like. The stricter regulations have made it impossible to meet regulatory values only by improving the engine. Thus, there has been employed a technology of reducing the above-described substances emitted from the engine by installing an exhaust gas treatment system.
Meanwhile, for NOx, a number of NOx purification catalysts have been developed. Moreover, for PM, a filter called a diesel particulate filter (hereinafter abbreviated as DPF) has been developed.
One of the NOx purification catalysts is an NOx occlusion/reduction type catalyst. This NOx occlusion/reduction type catalyst is formed by carrying catalytic metal and an NOx occlusion material having an NOx occlusion function on a porous catalyst coat layer such as alumina (Al2O3). The catalytic metal is platinum (Pt) or the like, which has an oxidation function for NOx. Moreover, the NOx occlusion material is any one of or a combination of some of the following: alkali metal such as sodium (Na), potassium (K) and cesium (Cs); alkali earth metal such as calcium (Ca) and barium (Ba); rare earth such as yttrium (Y) and lanthanum (La); and the like. The NOx occlusion/reduction type catalyst exerts two functions which are NOx occlusion and NOx emission/purification, depending on an O2 (oxygen) concentration in the exhaust gas.
First, under an exhaust gas condition with a high O2 concentration in the exhaust gas (a lean air fuel ratio state), such as a normal operating state of the diesel engine, a lean-burn gasoline engine or the like, the oxidation function of the catalytic metal allows released NO (nitrogen monoxide) to be oxidized by O2 contained in the exhaust gas, thereby generating NO2 (nitrogen dioxide). This NO2 is occluded in the form of a chloride by the NOx occlusion material. Therefore, the exhaust gas is cleaned.
However, if the occlusion of NOx is continued, the NOx occlusion material such as barium is changed into a nitrate and gradually saturated to lose the function of occluding NO2. Therefore, excessive concentration combustion is performed by changing engine operating conditions, and thereby rich spike gas is generated and supplied to the catalyst. This rich spike gas is exhaust gas having a low O2 concentration, a high CO concentration, and a high exhaust gas temperature.
In the rich air fuel ratio state of the exhaust gas described above, the NOx occlusion material, which has been changed into the nitrate by occluding NO2, releases the occluded NO2 and is changed back into its original form such as barium. Since no O2 exists in the exhaust gas, the released NO2 is reduced on the catalytic metal by use of CO, HC and H2 in the exhaust gas as a reducing agent, and is converted into N2, H2O and CO2. Therefore, the exhaust gas is cleaned.
However, in the case of using the NOx occlusion/reduction type catalyst, the catalyst cannot burn soot components in the PM by itself. Consequently, it is required to combine the catalyst with the DPF or to integrate an NOx purification function of the NOx occlusion/reduction type catalyst with a PM purification function of the DPF.
As one of the integration described above, an exhaust gas cleaning device for an internal combustion engine has been proposed in Japanese patent application Kokai publication No. 1994-159037. Specifically, the exhaust gas cleaning device is intended to provide a means for reducing an amount of energy supplied from the outside in order to regenerate a DPF, and for facilitating ignition of collected PM. In this device, the DPF and an NOx absorbent (the NOx occlusion/reduction type catalyst) are disposed in positions where heat can be transmitted to each other. To be more specific, the DPF carries the NOx absorbent.
However, in an exhaust gas cleaning system including the NOx purification function of the NOx occlusion/reduction type catalyst and the PM purification function of the DPF, what matter is a balance between an NOx purification rate and a PM accumulation amount.
Specifically, in order to increase the NOx purification rate in a high load engine operating range at an exhaust gas temperature at which the PM is self-ignited as shown in FIG. 6, a frequency of rich combustion by which NOx is released and reduced may be increased. However, since the operating range described above is a high combustion temperature range, a large amount of PM is generated in the rich combustion. Note that the exhaust gas temperature in the self ignition described above, although influenced by a catalyst carried by the DPF, is approximately 300° C. to 400° C. or higher.
In this case, if the amount of PM generated is less than a rate at which the PM is self-ignited and burnt, the NOx purification rate is reduced although no PM is accumulated in the DPF. Meanwhile, in the case where the amount of PM generated is increased through increasing richness (a frequency, a degree of richness, an amount of a reducing agent supplied, and the like) in order to increase the NOx purification rate, an amount of PM accumulated in the DPF is gradually increased. If the PM accumulation amount exceeds its limits, there arises a risk of abnormal combustion and erosion attributable thereto. Thus, even if the engine operating range is the high load range, it is required to regenerate the DPF by forcibly burning the PM.
FIG. 7 shows a relationship between the NOx purification rate and the PM accumulation amount (a differential pressure increasing rate). When the richness is reduced by a catalyst regenerative control for recovering the NOx purification rate of the NOx occlusion/reduction type catalyst, the amount of PM generated becomes smaller than the amount of PM burnt (the differential pressure increasing rate is negative). However, the NOx purification rate is also reduced. Meanwhile, when the NOx purification rate is increased through increasing the richness, the amount of PM generated becomes larger than the amount of PM burnt (the differential pressure increasing rate is positive) although the NOx purification rate increases.